The present invention is directed at an improved method and apparatus for measuring the quantity of a liquid or gas through a flow meter. Specifically, the present invention provides a scaled quadrature pulse output signal from an unscaled flow meter input signal.
Rotary motion flowmeters include all positive displacement (PD) and turbine types. Also included in this category are metering pumps of many types. Almost all rotary meters today are of the type where the rotary motion of the meter is nominally proportional to the rate of flow through the meter and the motion is converted to a set of pulses using a rotary shaft encoder. Flowmeters that meet these criteria include gasoline dispenser meters, household water meters, and natural gas meters.
Rotary motion flowmeters have existed for many years, the most common of which is the PD flowmeter. PD flowmeters use the rotational displacement of the shaft to represent a fixed volume per revolution and are equipped with mechanical counting devices that provide a readout of the volume of flow that passes through the meter. (A mechanical counter is a series of wheels with the numbers 0 through 9 arranged in decades such that the lesser significant digit wheel increments the next most significant digit wheel by one count each time it completes one full revolution. A common example of such a mechanical counter is the odometer of an automobile.) Because of different displacement volumes of different size flowmeters, in order to calibrate the mechanical counter to readout in meaningful flow units such as gallons or liters, a gear train is typically used to reduce or increase the speed of rotation to the counter. If high precision of the flowmeter is needed, a fixed nominal gear ratio is insufficient to account for differences between meters due to variations in the manufacturing process. Individual meter calibration and a fine tuning mechanism must be put in place to accomplish high accuracy calibration to meet weights and measures metering standards (on the order of plus or minus 0.2% of actual). Even with a fine tuning mechanism to initially calibrate the flowmeter, the meter may be accurate over only a limited range of flowrates because of its non-linear behavior over a wider range.
Both forward and reverse counting is possible with many flowmeters equipped with mechanical counters. A symmetrical PD flowmeter can be flowed through in either and/or both directions counting up during forward flow and back down during reverse flow.
With the advent of electronics, the need to convert the rotary motion of the PD meter into electronic signals became apparent. Replacing the mechanical counter with a rotary pulse encoder of some type became commonplace. Initially, the calibration of the pulses, which could be counted or xe2x80x9ctotalizedxe2x80x9d, was accomplished using gearing where the gear ratio of the pulse encoder could be adjusted to make each pulse represent a known unit of volume. In order to distinguish between forward and reverse flow, the most common technique uses a rotary encoder with two pulse output channels where one channel is offset from the other by 90 degrees in phase relationship. When channel A leads channel B, the counting electronic circuit interprets the flow as forward and counts up. When channel B leads channel A, the counter interprets the flow as reverse and counts down. This is known as a quadrature encoder.
When digital logic and microprocessors became popular, the need for gearing the raw encoded signal was reduced because pulse weighting could be handled by scaling the received counts by a mathematical scaling factor to obtain the desired output result. Many electronic counting registers exist that receive unscaled flowmeter pulses from a pulse encoder then apply a calibration factor or pulse weighting factor to the received count.
Despite such advances in the art of flow meters, there has not been provided an electronic flow meter that produces a scaled quadrature pulse output signal.
The present invention is directed at an apparatus and method for providing an accurate measurement of the flow rate of a liquid or gas under varying conditions. In general, the product flow generates a series of unscaled pulses. The pulses are applied to a pulse scaler that applies a weight factor to each pulse based upon select conditions. The scaled pulse weight is added to or subtracted from a fractional totalizer. Preferably, the fractional totalizer is in binary format having a half-bit and a quarter-bit for indicating xc2xc, xc2xd, xc2xe, and 1 unit of product volume.
Preferably, a rotary shaft encoder provides a plurality of pulses per unit of product volume. The number of pulses produced per unit volume defines the maximum resolution for volume measurement. For example, the rotary shaft encoder may provide approximately 1000 pulses per gallon of product. The encoder pulses are dependent upon rotary motion only and are thus unscaled. The unscaled pulses are preferably next scaled by applying a weight factor to each pulse. For example, as temperature decreases, the weight factor may increase the weight for each pulse by a percentage. The weight factor is applied to the pulse weight to obtain a scaled pulse. For example, an unscaled pulse may represent {fraction (1/1000)} (0.001) of a gallon. Based upon one or more conditions, the weight factor may increase the pulse volume representation by a 1.0005 multiplier. Thus, in this example, the scaled pulse will represent 0.0010005 gallon. A totalizer receiving the pulse adds (or subtracts, depending upon flow direction)0.0010005 to the volume total as each pulse is received, provided the weight factor remains unchanged. A totalizer value-tester represents the volume total in binary format and provides a two bit output representing when the volume total is at or over xc2xc, xc2xd, xc2xe, or 1 gallon. The two bit output may be provided to a counter system and display device for providing a visual representation of the volume total.
The two bit output also indicates the direction of product flow. For example, if the half-bit and quarter-bit combination change from 0 0 to 0 1, then the product flow is moving in the positive direction. However, in this example if the half-bit and quarter-bit combination change from 0 0 to 1 1, then the product flow is moving in the negative direction.
Thus, the rotary motion flowmeter produces a scaled quadrature pulse output signal that essentially reacts in real time. This technique allows the pulse train to contain direction of flow information and allows scaled real time pulses to be temperature volume compensated. Through this method, the pulses may also be scaled non-linearly to correct any inherent non-linearity in the measurement system thus producing a linear result.